Electronically tunable acousto-optic bandpass filters have been constructed so that a cone of light of first polarization is diffracted by an acoustic wave in an anisotropic medium (such as a birefringent crystal) to shift the light beam from the first to a second polarization at a selected bandpass of optical frequencies. The center wavelength of the passband of the acousto-optic filter is electronically tunable by changing the frequency of the acoustic wave within the crystal.
Two basic types of tunable acousto-optic filters have been constructed: collinear and noncollinear. In the collinear filter, the incident and diffracted light beams inside the birefringent crystal are collinear with the acoustic beam. The diffracted light beam at the selected passband is separated from the incident light beam by crossed polarizers. The collinear type of acousto-optic filter is disclosed in an article entitled "Acousto-Optic Tunable Filters" appearing on pages 744-747 in the June, 1969, issue of The Journal of the Optical Society of America (Vol. 59, No. 6), and in U.S. Pat. No. 3,679,288, entitled "Tunable Acousto-Optic Method and Apparatus."
In the noncollinear filter, the light beams inside the birefringent crystal are noncollinear with the acoustic beam. The diffracted light beam at the passband is separated from the incident light beam by either crossed polarizers or spatial separations. The noncollinear type of acousto-optic filter is disclosed in an article entitled "Noncollinear Acousto-Optic Filter with Large Angular Aperture," appearing on pages 370-372 of the Oct. 15, 1974, issue of the Applied Physics Letters (Vol. 25), and in U.S. Pat. No. 4,052,121, entitled "Noncollinear Tunable Acousto-Optic Filter."
The most significant feature of both the collinear and noncollinear tunable acousto-optic filter is that a narrow filter bandpass can be maintained for a relatively large cone of incident light. This large angular aperture characteristic is due to the proper choice of acousto-optic interaction geometry, wherein the tangents to the locus of the incident and diffracted light wave vectors are parallel. This condition of "parallel tangents" applies to both noncollinear and collinear acousto-optic filters.
For the remainder of this disclosure, "tunable acousto-optic filter" will be defined as an optical filter that operates on the basis of the "parallel tangents" type of acousto-optic diffraction in an anisotropic medium wherein the "parallel tangents" condition is satisfied. This type of tunable acousto-optic filter is clearly distinguishable from the type with small angular aperture as disclosed in U.S. Pat. Nos. 3,944,334, 3,944,335 and 3,953,107, all of which are entitled "Acousto-Optic Filters."
One of the principal disadvantages of a tunable acousto-optic filter is that its optical aperture is limited by the driving acoustic power. The problem is particularly severe for the infrared region since the drive power is proportional to the square of wavelength. One possible method to reduce the drive power is to operate the filter in an acoustic resonator configuration. At a frequency corresponding to one of the acoustic resonances, the acoustic energy inside the resonator is many orders of magnitude higher than that of a single pass traveling acoustic wave and as such the required external drive power is reduced. Despite this advantage, however, operating tunable acousto-optic filters using acoustic resonance has been considered impractical. Maximum filter transmission occurs only at discrete wavelengths that correspond to acoustic resonance peaks. When the passband wavelength of the acousto-optic filter is tuned by changing the acoustic frequency, the filter spectral responses are in the form of multiple resonances or combs. Such amplitude variation in the filter spectral response is undesirable, and is eliminated in previous art by damping acoustic resonance in the filter medium. Acousto-optic filters having means for damping acoustic resonances are disclosed in U.S. Pat. Nos. 3,729,250 and 3,767,286.
An acoustically resonant tunable acousto-optic filter with smoothly tuning spectral response based on electro-optic tuning was described in U.S. Pat. No. 3,701,584. The acoustic frequency is fixed at one of the acoustic resonance peaks to reduce the drive power. Tuning of the filter is accomplished by varying the birefringence of the filter crystal with the application of a voltage on the crystal. This method is limited to acousto-optic filter crystals that are also electro-optic. Moreover, the tuning range is limited due to the large tuning voltage required.